NHibernate is a mature, open source object-relational mapper (ORM) for the .NET framework.

This article is dedicated to discussing the latest releases of the NHibernate and Entity Framework. NHibernate is (was?) a number one ORM Framework for ages. Its feature list is growing and some things such as code-based mappings and automatic migrations have found a place in Entity Framework. Moreover, it is now open-source, so they are now accepting pull requests as well.